Airborne electromagnetic survey system with multiple transmitter arrangements

ABSTRACT

An airborne electromagnetic survey system includes: a transmitter arrangement comprising at least one current source configured to connect with a transmitter coil arrangement, and a switch arrangement between the at least one current source and the transmitter coil arrangement, the switch arrangement configured to (1) connect the at least one current source with the transmitter coil arrangement to build up a connecting secondary electromagnetic field, and (2) disconnect the at least one current source from the transmitter coil arrangement to build up a disconnecting secondary electromagnetic field; and a receiver arrangement comprising at least one receiver coil; wherein the airborne electromagnetic survey system further comprises an additional transmitter coil arrangement and a switch arrangement controller, the switch arrangement controller configured to disconnect the at least one current source from the transmitter coil arrangement and the additional transmitter coil arrangement at substantially a same time.

RELATED APPLICATION DATA

This application is a continuation of U.S. patent application Ser. No.15/126,869 filed Sep. 16, 2016, pending, which is the national phase ofInternational Patent Application No. PCT/DK2015/050051, filed Mar. 16,2015, which claims priority to Danish Patent Application No. PA 201470130, filed Mar. 17, 2014. The entire disclosures of all of the aboveapplications are expressly incorporated by reference herein.

FIELD

An objective is achieved by an airborne electromagnetic survey systemconfigured for geophysical prospecting of an underground formation,which airborne electromagnetic survey system comprises a transmitterarrangement T_(x) comprising at least one current source connected to atleast one transmitter coil arrangement covering an area A for generatinga magnetic moment for building up a primary electromagnetic fieldinducing a secondary electromagnetic field in the underground formationand a switch arrangement comprising at least one switch inserted inbetween the at least one current source and the at least one transmittercoil arrangement.

The switch arrangement is configured to connect the at least one currentsource with the at least one transmitter coil arrangement to build up aconnecting secondary electromagnetic field in a underground formation,which connect is at a connection time at a connection voltage at aconnection current. The switch arrangement is furthermore arranged todisconnect the at least one current source from the at least onetransmitter coil arrangement to build up a disconnecting secondaryelectromagnetic field in a underground formation; which disconnect is ata disconnection time at a disconnection voltage at a disconnectioncurrent. The connecting secondary electromagnetic field anddisconnecting secondary electromagnetic fields form a combined secondaryelectromagnetic field.

BACKGROUND

Geophysical prospecting by application of electromagnetic surveys wherea primary electromagnetic field is generated to induce a secondaryelectromagnetic field in an underground formation has become widespread.During the last decades, survey systems for performing such prospectinghave become airborne. An airborne electromagnetic system may comprise atow assembly connected to an aircraft, typically a helicopter, andfurther comprising a transmitter system for generating the primaryelectromagnetic field that induces the secondary electromagnetic fieldin the underground formation. The secondary electromagnetic field isdetected by a receiver system.

Achieving a high magnetic moment has been a goal achieved by increasingthe current or the area of the transmitter. Such straight forwardimprovements have previously resulted in larger frames or larger currentsources, which have proven difficult to implement or operate in airbornesystems.

Larger frames with larger transmitter coils and thus a larger area haveproven much more difficult to handle operationally than foreseen.

Simply increasing the current has also proven difficult due toappearance of higher order electromagnetic effects becoming significantand thus imposing limitation on how early measurements can be made intransient electromagnetic (TEM) systems. This implies longer wait times,basically a “dead-time” before the current has decayed in a transmittercoil. One way to reduce the wait time is to use a conductor or wirewhere each core is isolated, such as a Litz Wire. It is an objective toable to measure at event earlier times or to provide an alternative waythan using special conductors.

Likewise, increasing the number of turns in a transmitter coil hasproven difficult due to higher voltages when switching on and off thecurrent source from the transmitter coil. Higher voltages may causedevastating and dangerous voltage breakthroughs or simply reduce qualityof the measurements.

U.S. Pat. No. 5,796,253 relates to time domain electromagnetic mappingtechnique for geophysical prospecting and discloses techniques where asequence or succession of multiple half sinusoids transmitter pulsesreplaces a single half sinusoid waveform to achieve steeper decays ofthe or fall time of the current in the transmitter coil. The letterdiscloses circuits for generating sequences or pulse trains of such halfsinusoids in uncoupled transmitter coils.

It is an objective of this disclosure to improve upon such limitations.

SUMMARY

An objective is achieved by an airborne electromagnetic survey systemconfigured for geophysical prospecting of an underground formation,which airborne electromagnetic survey system comprises a transmitterarrangement T_(x) comprising at least one current source connected to atleast one transmitter coil arrangement covering an area A for generatinga magnetic moment for building up a primary electromagnetic fieldinducing a secondary electromagnetic field in the underground formationand a switch arrangement comprising at least one switch inserted inbetween the at least one current source and the at least one transmittercoil arrangement.

The switch arrangement is configured to connect the at least one currentsource with the at least one transmitter coil arrangement to build up aconnecting secondary electromagnetic field, which connect is at aconnection time at a connection voltage at a connection current. Theswitch arrangement is furthermore arranged to disconnect the at leastone current source from the at least one transmitter coil arrangement tobuild up a disconnecting secondary electromagnetic field; whichdisconnect is at a disconnection time at a disconnection voltage at adisconnection current. The connecting secondary electromagnetic fieldand disconnecting secondary electromagnetic field provide or form acombined secondary electromagnetic field.

The airborne geophysical prospecting system further comprises a receiverarrangement R_(x) comprising at least one receiver coil configured toreceive the combined connecting and disconnecting secondaryelectromagnetic fields. The airborne electromagnetic survey systemcomprises multiple transmitter arrangements T_(x) generating a totalsecondary electromagnetic field comprising a sum of individual combinedsecondary electromagnetic fields.

Thereby is achieved a significant increase in the magnetic moment byapplying several transmitter arrangements T_(x).

“Connect” or “disconnect” may be understood an event or an action.

Alternatively, a much smaller frame can be used and thus allow foroperation either at places or during circumstances otherwise excluded orcumbersome. Furthermore, using smaller systems or frames will result infaster survey times and/or at reduced operational costs.

One effect is that the turn off or disconnect voltage in this manner canbe reduced for the individual transmitter coil arrangement, whilstmaintaining the turn off time, and thereby reduce the risk for currentpenetration and hazard for operation personal.

Another effect or advantage is that smaller coils or thinner wires canbe used to achieve the same magnetic moments without the disadvantagesof otherwise larger coils or thicker wires required to carry requiredcurrents and/or to mitigate undesirable electromagnetic effects such ascoupling, self-inductance and/or eddy currents.

A transmitter arrangement may have separate transmitter coils. Thetransmitter coils may be in series, in parallel or in groups in parallelbeing in series and/or in groups in series being in parallel.

In an embodiment, the airborne electromagnetic survey system may furthercomprise a switch arrangement controller configured to control eachswitch arrangement of the system.

Thus, the electromagnetic system will be able to control each switcharrangement according to operational circumstances and provide a meansto perform switching as precisely as required.

The switch arrangement controller may be programmable or hard wired withadjustable timing means for providing timely control signals. In anembodiment the switch arrangement controller may be connected to eachswitch arrangement by cables adjusted in length or otherwise tocompensate for time of flight of signals from the location of thecontroller to each switch arrangement.

In an embodiment of the airborne electromagnetic survey system, theswitch arrangement controller and each switch arrangement is configuredto control each switch arrangement to disconnect each current sourcefrom each transmitter coil arrangement at substantially the same time.

By disconnecting at the same time is understood that each switch isconfigured to be switched to disconnect the current source from thetransmitter coil generating the secondary field contribution so that thecharacteristics of the secondary field is desired.

Ideally the switch arrangements should disconnect at the same time orsimultaneously.

The secondary field contributions should in practice be synchronised towithin 50 μs, to within 10 μs; or more preferably to within 1 μs.

In an embodiment where transmitter coils and transmitter arguments areidentical, the controller may be arranged to provide switching signals,and switching arrangements may be configured to switch individualswitches to within 50 μs, preferably within 10 μs or more preferably towithin 1 μs.

A starting point may be to use identical components extending to thelength of cables. Alternatively, each transmitter arrangement may beadjusted so as to synchronize. Such synchronisation may requireintroduction of delays in components, in between components oradjustment of the signals from the controller.

A person skilled in the art will acknowledge the need for somesystematic calibration and synchronising work.

A further advantage of disconnecting to the same time or simultaneouslyis that the electromagnetic field generated or induced when the systemis synchronised will be as pure or identical as possible and thus resultin less need for post-processing of measured data.

In an embodiment of the airborne electromagnetic survey system, theswitch arrangement controller is configured to record each disconnectiontime of each disconnect by each switch arrangement.

To further improve or advance the survey system or to compensate fortime lags or time differences, recording of the time of disconnectiontime of each disconnect has been observed to be valuable. The time ofdisconnect may be provided by measuring currents or voltages or fieldsoccurring from the transmitter coils. Alternatively, recordings ofcontroller signals may provide measurements of disconnection time. Ineither case, the times of each disconnection time may be recorded andstored along with recordings from the receiver arrangement.

This will allow for measured data to be post processed and takingeffects of time differences into account in a corrective fashion.

In an embodiment of the airborne electromagnetic survey system, theswitch arrangement controller and each switch arrangement is configuredto control each switch arrangement to connect each current source toeach transmitter coil arrangement at substantially the same time.

Configuration of the controller is essentially the same as for thedisconnection outlined. However, the person skilled in the art willappreciate differences in current management when disconnecting and whenconnecting. Attempts to apply simple configurations that are identicalor symmetrical for providing current and leading currents away mayresult in undesirable results such as remaining electromagnetic fields.

In an embodiment the controller is configured to ramp up the current ina transmitter coil arrangement directly by connecting a voltage sourcethrough a connecting switch and to let the current ramp up—essentiallyexponentially—to the more or less maximal value in steady state givenOhm's law as the voltage across a transmitter coil over the resistancein the transmitter coil.

In another embodiment the current in a transmitter coil is cut-off andused to charge a capacitor as long as charge from the transmitter coilis available. The remaining current is drained by a resistor. The storedcharge in the capacitor is then used to ramp up the current in atransmitter coil again and the high voltage results in a very fastramp-up of the current. When the voltage across the capacitor has beenlowered to a given value, the connection between the transmitter coiland the capacitor is switched off and a connection between thetransmitter coil and a generator is switched on, possibly through acapacitor. The generator then maintains the current until the nextswitch-off or cycle.

This switching method has the advantage that is ramps-up the current inthe transmitter coil very fast. Likewise this switching method canramp-down the current equally fast.

In an embodiment of the airborne electromagnetic survey system, theswitch arrangement controller may be configured to record eachconnection time of each connect by each switch arrangement.

Likewise, recording of the connections may be advantageous. A personskilled in the art implements the same type of logic in recording thedisconnect and connect based on the logic of a controller. Makingrecording of changes in the currents or voltages or fields in thetransmitter arrangements may require different implementations of thesensing of disconnect and connect due to differences in the voltages orcurrents during switching off and during switching on.

Besides the logic of the controller, the switch arrangements includingactual switches may be properly configured. A switch arrangement may beconfigured to handle disconnect voltages of between 0-6 kV. Inparticular, a switch arrangement may be configured to handle voltagebreakthroughs from 500 V, 1800 V upward to about 5 kV.

Also, the switch arrangements may be configured to operate withdisconnect currents below about 100 mA. Ideally, the arrangement may beconfigured to disconnect with zero amperage.

Likewise, the switch arrangements may be configured to connect atvoltages of between 1 V to 5 kV. Also the switch arrangements may beconfigured to connect currents in the order of 5 A, or even in the rangeof 50 A to 500 A.

The lower voltage in the order of 1 V may be applicable when the currentramp-up is direct and the larger voltage is when the generator may takeover or when switched from a capacitor to a generator.

Ideally, the system may be able to connect at as high amperage aspossible and disconnect to zero amperage without any effect of thevoltage. However, the above mentioned ranges have been observed to beadequate in practice and be configurable by using available componentsat tolerable sizes and weights of equipment. This to a degree whereconfiguring a system using multiple transmitter arrangements becomeadvantageous or necessary to overcome deficiencies of the prior art.

In an embodiment of the airborne electromagnetic survey system, there isonly a single current source.

This embodiment may be advantageous since it allows for relativelysimple configuration where power handling is required only at one placeor in one system.

In an embodiment a power generator feeds one or more current sources.The power generator may feed the current sources in parallel, in seriesor sequentially.

In an embodiment of the airborne electromagnetic survey system, acurrent source comprises a capacitor, a super capacitor, or a battery;or a bank of such.

Capacitors are advantageous since they can be easily recharged andconfigured to deliver a certain quantified amount of power.

Super capacitors or ultra capacitors are further advantageous due to thehigher energy storage as compared to capacitors.

In an embodiment of the airborne electromagnetic survey system, thecurrent source comprises a generator being a motor generator or a fuelcell.

In an embodiment mechanical energy storage may be used in between atransmitter arrangement and a generator. In an embodiment the mechanicalenergy storage is a flywheel configuration. An advantage of a flywheelis that the weight may be reduced compared to a capacitor/supercapacitor embodiment. A further advantage may be that otherwisecomplicated electronics may be eliminated.

The generator may be configured to charge or recharge capacitors orbatteries or combinations thereof. A person skilled in the art willappreciate the design options provided by a Ragone representation toconfigure that current source according to operational requirements interms of power requirements, i.e. energy density and power density.Whilst conventional capacitors may have a high power density (W/kg) andfast charge and de-charge times (in the order of μ-seconds),conventional capacitors may have a low energy density. Ultra capacitorswill have slower charge and de-charge times (eg. seconds), but higherenergy densities. Batteries in the other end will have charge time ofhours much higher energy densities. Finally, fuel cells (including fuel)will have even higher energy densities, but lower power densities, aswill motor generators.

In an embodiment of the airborne electromagnetic survey system, a switcharrangement comprises at least two switches.

In an embodiment, each switch may be connected to a transmitterarrangement and the control of switches may be as follows. First aconnection from a first transmitter coil to a current source isestablished and the current is cut off. Second a connection from asecond transmitter coil to a current source is established and thecurrent is cut off. Thus one is active and another is inactive. This isrepeated. In this embodiment the transmitter coils may be placed on topof each other to couple the two coils. The direction of currents may beopposite in the two coils and thus the primary magnetic field can bereversed. Such configuration may require less electronics and less powerlosses. Compared to the double switch arrangement, two transmitter coilsare required and thus the system weighs more.

In another embodiment four switches in a bridge configuration may beused. Compared to the double switch arrangement, only transmitter coilsis required and thus the system weighs less.

Thereby is provided a switch for connection and a switch fordisconnection.

In an embodiment of the airborne electromagnetic survey system, a switcharrangement is arranged with switches in a bridge configuration.

Thereby is provided a switch arrangement type that allows for connectionas well as disconnection and in particular handling currents to achieveeven sharper or more precise transitions.

In an embodiment of the airborne electromagnetic survey system, a switchis a semiconductor type switch. Although the working of the disclosedswitch arrangement may use other types of switches such as contacts orconfigurations of contacts, a semiconductor type switch has showed toprovide adequate switching times and characteristics as well as beingeasily controllable.

In an embodiment of the airborne electromagnetic survey system, thetransmitter coil arrangements are placed on top of each other thusproviding an essentially 100% coupling between the transmitter coils.

In an embodiment of the airborne electromagnetic survey system, eachcombined secondary electromagnetic fields are essentially identical.

The placement of a receiver arrangement in relation to transmitterarrangements may be performed according to design. The receiverarrangement may be located in the centre of the transmitter arrangement.The receiver arrangement may be in the same plane or shifted to anotherplane. Alternatively, the receiver arrangement may be placed in alocation where the magnetic field strength from the transmitter coil isminimal if not eliminated.

In an aspect, an objective is achieved by a method of geophysicalprospecting comprising use of an airborne electromagnetic survey systemconfigured for geophysical prospecting of an underground formation. Themethod of geophysical prospecting comprises generating a total secondaryelectromagnetic field comprising a sum of individually combined(connecting and disconnecting) secondary electromagnetic fields usingmultiple transmitter arrangements T_(x). A transmitter arrangement maycomprise at least one current source connected to at least onetransmitter coil arrangement covering an area A for generating amagnetic moment for building up a primary electromagnetic field inducinga secondary electromagnetic field in the underground formation. This maybe achieved by switching a switch arrangement comprising at least oneswitch inserted in between the at least one current source and the atleast one transmitter coil arrangement and configured to connect the atlast one current source with the at least one transmitter coilarrangement to build up a connecting secondary electromagnetic field,which connect is at a connection time at a connection voltage at aconnection current. The switching arrangement may furthermore beconfigured to disconnect the at least one current source from the atleast one transmitter coil arrangement to build up a disconnectingsecondary electromagnetic field; which disconnect is at a disconnectiontime at a disconnection voltage at a disconnection current.

The method of geophysical prospecting may also comprise receiving acombined, connecting and disconnecting, secondary electromagnetic fieldusing a receiver arrangement R_(x) comprising at least one receiver coilconfigured to receive the combined, connecting and disconnecting,secondary electromagnetic fields.

In a further embodiment a method of geophysical prospecting encompassesswitching of each switch arrangement that comprises disconnecting the atleast one current source from the at least one transmitter coilarrangement to build up a disconnecting secondary electromagnetic field;which disconnecting is at a disconnection time at a disconnectionvoltage at a disconnection current; which disconnecting of each currentsource from each transmitter coil arrangement is at substantially thesame time.

In a further embodiment a method of geophysical prospecting encompassesswitching of each switch arrangement that comprises connecting the atleast one current source to the at least one transmitter coilarrangement to build up a connecting secondary electromagnetic field;which connecting is at a connection time at a connection voltage at aconnection current; which connecting of each current source to eachtransmitter coil arrangement is at substantially the same time.

The methods solve the same problems as described for the systems.Furthermore, a person skilled in the art will appreciate that furthersteps or processes in the method can be accomplished by making use ofsystem features as disclosed, or equivalents. As such, a person skilledin the art will appreciate transforming a system feature to a methodstep having the same functionality. As such carrying out steps may notbe limited to making use of the herein disclosed system components, butalso be achieved by performing steps having the same or equivalentfunctionality.

An airborne electromagnetic survey system configured for geophysicalprospecting of an underground formation, the airborne electromagneticsurvey system includes: a transmitter arrangement comprising at leastone current source configured to connect with a transmitter coilarrangement covering an area for generating a magnetic moment forbuilding up a primary electromagnetic field, and a switch arrangementcomprising at least one switch between the at least one current sourceand the transmitter coil arrangement, the switch arrangement configuredto (1) connect the at least one current source with the transmitter coilarrangement at a connection time at a connection voltage at a connectioncurrent, to build up a connecting secondary electromagnetic field, and(2) disconnect the at least one current source from the transmitter coilarrangement at a disconnection time at a disconnection voltage at adisconnection current, to build up a disconnecting secondaryelectromagnetic field, wherein the connecting secondary electromagneticfield and the disconnecting secondary electromagnetic field provide acombined secondary electromagnetic field; and a receiver arrangementcomprising at least one receiver coil configured to receive the combinedsecondary electromagnetic field; wherein the airborne electromagneticsurvey system further comprises an additional transmitter coilarrangement and a switch arrangement controller, the switch arrangementcontroller configured to disconnect the at least one current source fromthe transmitter coil arrangement and the additional transmitter coilarrangement at substantially a same time for generating individualsecondary electromagnetic fields whose sum contributes to a totalsecondary electromagnetic field.

Optionally, the transmitter coil arrangement and the additionaltransmitter coil arrangement are coupled to each other by being on topof each other.

Optionally, the switch arrangement controller is configured to recordthe disconnection times associated with the respective transmitter coilarrangements.

Optionally, the switch arrangement controller is also configured toconnect the at least one current source to the transmitter coilarrangement and the additional transmitter coil arrangement atsubstantially a same time.

Optionally, the individual combined secondary electromagnetic fields areidentical to each another.

Optionally, the switch arrangement controller is configured to recordthe connection times associated with the respective transmitter coilarrangements.

Optionally, the at least one current source comprises only a singlecurrent source.

Optionally, the current source comprises a capacitor, a super capacitor,a mechanical storage, a flywheel, a battery, or any combination of theforegoing.

Optionally, the current source comprises a motor generator or a fuelcell.

Optionally, the switch arrangement comprises least two switches.

Optionally, the switch arrangement comprises switches in a bridgeconfiguration.

Optionally, the switch is a semiconductor type switch.

A method of geophysical prospecting comprising use of an airborneelectromagnetic survey system configured for geophysical prospecting ofan underground formation, the method includes: generating a totalsecondary electromagnetic field comprising a sum of individual combinedsecondary electromagnetic fields, wherein the act of generating thetotal secondary electromagnetic field comprises: switching, via a switcharrangement comprising at least one switch between at least one currentsource and a transmitter coil arrangement, to (1) connect the at leastone current source with the transmitter coil arrangement at a connectiontime at a connection voltage at a connection current, to build up aconnecting secondary electromagnetic field, and (2) disconnect the atleast one current source from the transmitter coil arrangement at adisconnection time at a disconnection voltage at a disconnectioncurrent, to build up a disconnecting secondary electromagnetic field;and receiving the connecting and disconnecting secondary electromagneticfield using a receiver arrangement comprising at least one receivercoil; wherein the act of switching further comprises disconnecting theat least one current source from an additional transmitter coilarrangement, and wherein the transmitter coil arrangement and theadditional transmitter coil arrangement are disconnected from the atleast one current source at substantially a same time.

Optionally, the act of switching further comprises connecting the atleast one current source to the additional transmitter coil arrangement,wherein the transmitter coil arrangement and the additional transmittercoil arrangement are connected to the at least one current source atsubstantially a same time.

Optionally, the transmitter coil arrangements are coupled to each otherby being on top of each other.

Other and further aspects and features will be evident from reading thefollowing detailed description of the embodiments.

BRIEF DESCRIPTION OF DRAWINGS

Embodiments will be described in the figures, whereon:

FIG. 1 illustrates an airborne electromagnetic system for geophysicalprospecting of an underground;

FIG. 2 illustrates a transmitter arrangement for inducing a secondaryelectromagnetic field in the underground and a receiver arrangement forreceiving a combined secondary electromagnetic field;

FIG. 3 illustrates a transmitter arrangement and a switch arrangementfor connecting and disconnecting a transmitter coil to a current sourcebuilding up a connecting secondary electromagnetic field and adisconnecting secondary electromagnetic field;

FIG. 4 illustrates N-multiple transmitter arrangements building up atotal secondary electromagnetic field;

FIG. 5 illustrates a controller arrangement configured to controlmultiple transmitter arrangements;

FIG. 6 illustrates a controller arrangement configured to control switchcontrols in a switch arrangement of a transmitter arrangement;

FIG. 7 illustrates switch arrangements; including a bridge arrangement;

FIG. 8 illustrates multiple transmitter arrangements, each with a singlecurrent source and a pair of switch arrangements and transmitter coils;and

FIG. 9 illustrates a method of geophysical prospecting using multipletransmitter arrangements.

DETAILED DESCRIPTION

Item No Airborne electromagnetic survey system 1 Geophysical prospecting2 Underground formation 3 Magnetic moment 5 Transmitter arrangement, Tx10 Primary electromagnetic field 11 Secondary electromagnetic field 12Current Source 15 Switch arrangement 20 Switch 22 Transmitter coilarrangement 24 Connect 30 Connection time 32 Connection voltage 34Connection current 36 Connecting secondary electromagnetic field 38Disconnect 40 Disconnection time 42 Disconnection voltage 44Disconnection current 46 Disconnecting secondary electromagnetic field48 Combined (connecting and disconnecting) secondary 50 electromagneticfield Total secondary electromagnetic field 55 Receiver arrangement 60Receiver coil 62 Switch arrangement controller 70 Switch control 72 Sametime 75 Generator 80 Bridge Configuration 90 Method of geophysicalprospecting 100 Generating 200 Switching 300 Connecting 320Disconnecting 340 Receiving 400

Various embodiments are described hereinafter with reference to thefigures. It should be noted that elements of similar structures orfunctions are represented by like reference numerals throughout thefigures. It should also be noted that the figures are only intended tofacilitate the description of the embodiments. They are not intended asan exhaustive description of the claimed invention or as a limitation onthe scope of the claimed invention. In addition, an illustratedembodiment needs not have all the aspects or advantages shown. An aspector an advantage described in conjunction with a particular embodiment isnot necessarily limited to that embodiment and can be practiced in anyother embodiments even if not so illustrated.

FIG. 1 illustrates an airborne electromagnetic system 1 for geophysicalprospecting 2 of an underground 3. The system is configured to provide amagnetic moment 5 by means of a transmitter arrangement 10.

FIG. 2 illustrates a transmitter arrangement 10 for inducing a secondaryelectromagnetic field 12 in the underground 3 and a receiver arrangement60 for receiving a combined secondary electromagnetic field 50.

The airborne electromagnetic survey system comprises a transmitterarrangement T_(x) 10 comprising at least one current source 15 possiblyfed by a generator 80 and connected to at least one transmitter coilarrangement 24 covering an area A for generating a magnetic moment 5 forbuilding up a primary electromagnetic field 11 inducing a secondaryelectromagnetic field 12 in the underground formation and a switcharrangement 20 (not shown).

The airborne geophysical prospecting system further comprises a receiverarrangement 60 R_(x) comprising at least one receiver coil 62 configuredto receive the secondary electromagnetic field 12 or the combinedconnecting and disconnecting secondary electromagnetic fields 50, wherethe connection and disconnection will be exemplified in the following.

FIG. 3 illustrates a transmitter arrangement 10 with a switcharrangement 20 for connecting and disconnecting a transmitter coilarrangement 24 to a current source 15 building up a connecting secondaryelectromagnetic field 38 and a disconnecting secondary electromagneticfield 48; both as induced secondary electromagnetic fields 12.

The airborne electromagnetic survey system comprises a transmitterarrangement 10 T_(x) comprising at least one current source 15 connectedto at least one transmitter coil arrangement 24 covering an area A forgenerating a magnetic moment 5 for building up a primary electromagneticfield inducing a secondary electromagnetic field 12 in the undergroundformation. The switch arrangement 20 comprises at least one switch 22inserted in between the at least one current source 15 and the at leastone transmitter coil arrangement 24.

The switch arrangement 20 is configured to connect 30 the at least onecurrent source 15 with the at least one transmitter coil arrangement 24to build up a connecting secondary electromagnetic field 38, whichconnect 30 is at a connection time 32 at a connection voltage 34 at aconnection current 36, which voltage and currents are illustrated by thepulse with a square pulse with an onset and an offset. The switcharrangement 20 is furthermore arranged to disconnect 40 the at least onecurrent source 15 from the at least one transmitter coil arrangement 24to build up a disconnecting secondary electromagnetic field 48; whichdisconnect 40 is at a disconnection time 42 at a disconnection voltage44 at a disconnection current 46. The connecting secondaryelectromagnetic field 38 and disconnecting secondary electromagneticfield 48 form a combined secondary electromagnetic field 50.

The airborne geophysical prospecting system further comprises a receiverarrangement R_(x) comprising at least one receiver coil configured toreceive the combined 50 connecting 38 and disconnecting 48 secondaryelectromagnetic fields 12.

Features described on FIG. 3 will be applicable to the followingfigures, without explicitly being shown in each figure.

FIG. 4 illustrates N-multiple transmitter arrangements 10I, 10II, . . .10N building up a total secondary electromagnetic field 55 bycontributions from combined secondary electromagnetic fields 50I, 50II,. . . 50N, respectively as induced due to individual magnetic moments5I, 5II, . . . 5N.

FIG. 5 illustrates a switch arrangement controller 70 configured tocontrol switch arrangements 20I, 20II, . . . 20N of multiple transmitterarrangements 10I, 10II, . . . 10N so as to generate magnetic moments 5I,5II, . . . 5N for inducing a total secondary electromagnetic field. Thecontrol to connect and to disconnect may be understood with reference toFIG. 3.

In particular the switch arrangement controller 70 and each switcharrangement 20I, 20II, . . . 20N are configured to control each switcharrangement 20I, 20II, . . . 20N to disconnect 40I, 40II, . . . 40N eachcurrent source 15I, 15II, . . . 15N from each transmitter coilarrangement 24I, 24II, . . . 24N at substantially the same time 75 (notshown). With reference to FIG. 3 this is to say that each disconnect40I, 40II, . . . 40N are essentially simultaneous.

Likewise, the switch arrangement controller 70 and each switcharrangement 20I, 20II, . . . 20N are configured to control each switcharrangement 20I, 20II, . . . 20N to connect 30I, 30II, . . . 30N eachcurrent source 15I, 15II, . . . 15N from each transmitter coilarrangement 24I, 24II, . . . 24N at substantially the same time 75 (notshown). With reference to FIG. 3 this is to say that each connect 30I,30II, . . . 30N are essentially simultaneous.

In an embodiment where transmitter coils and transmitter arguments areidentical, the controller may be arranged to provide switching signals,and switching arrangements may be configured to switch individualswitches to within 50 us, preferably within 10 us or more preferably towithin 1 us. As such it is understood that simultaneously or ‘at thesame time’ may be within a period of less than those times.

FIG. 6 illustrates a switch controller arrangement 70 configured tocontrol switch controls 72 in a switch arrangement 20 of a transmitterarrangement 10. In this particular embodiment there is a switcharrangement 20 with four individual switches (1), (2), (3) and (4),which individual switches are controlled by corresponding switchcontroller arrangements 70 control lines (1), (2), (3) and (4). In anembodiment with identical switch arrangements 20I, . . . 20N, thecontrol logic of the controller is to control individual switches (1),(2), (3) and (4) simultaneously. Possibly with adjustments to compensatefor delays in different switch arrangements 30.

FIG. 7 illustrates switch arrangements 20. The top figure shows a switcharrangement 20 with a switch 22 controlling a transmitter coilarrangement 24. The bottom figure illustrates a bridge arrangement 90with four individual switches 22 configured to connect 30 and disconnect40 the transmitter coil arrangement 24.

FIG. 8 illustrates multiple transmitter arrangement 10I, 10II, each witha single current source 15I, 15II, respectively, and each a pair ofswitch arrangement 201A, 201B and 2011A, 2011B and transmitter coilarrangements 241A, 241B and 2411A, 2411B, respectively.

In this embodiment identical switch arrangements 20 are used andswitches configured to operate to connect transmitter coils 241A, 241Bto the current source 15I at the same time as to connect transmittercoils 2411A, 2411B to the current source 15II. Similarly, the switcharrangements are configured to disconnect at the same time, but latertime than the connect time.

Furthermore, the figure illustrates the embodiment, where thetransmitter coils 241A, 241B, 2411A, 2411B are aligned on top of eachother to form essentially a coil structure spanning essentially the samespace.

FIG. 9 illustrates a method of geophysical prospecting 100 usingmultiple transmitter arrangements 10 (not shown).

The method 100 comprises steps of generating 200 a total secondaryelectromagnetic field.

The total secondary electromagnetic field 55 may compromise, withreference to previous figures, a sum of individual combined secondaryelectromagnetic fields 50 using multiple transmitter arrangements T_(x)10 which transmitter arrangement T_(x) 10 comprises at least one currentsource 15 connected to at least one transmitter coil arrangement 24covering an area A for generating a magnetic moment 5 for building up aprimary electromagnetic field 11 inducing a secondary electromagneticfield 12 in the underground formation 3.

This may be achieved by switching 300, again with references to previousfigures, a switch arrangement 20 comprising at least one switch 22inserted in between the at least one current source 15 and the at leastone transmitter coil arrangement 24 and configured to connect 30 the atlast one current source 15 with the at least one transmitter coilarrangement 24 to build up a connecting secondary electromagnetic field38, which connect 30 is at a connection time 32 at a connection voltage34 at a connection current 36; and to disconnect 40 the at least onecurrent source 15 from the at least one transmitter coil arrangement 24to build up a disconnecting secondary electromagnetic field 48; whichdisconnect 40 is at a disconnection time 42 at a disconnection voltage44 at a disconnection current 46.

The method 100 also encompasses receiving 400 a combined connecting anddisconnecting secondary electromagnetic field 50 using a receiverarrangement R_(x) 60 comprising at least one receiver coil 62 configuredto receive the combined connecting and disconnecting secondaryelectromagnetic fields 50.

Although particular embodiments have been shown and described, it willbe understood that they are not intended to limit the presentinventions, and it will be obvious to those skilled in the art thatvarious changes and modifications may be made without departing from thespirit and scope of the claimed inventions. The specification anddrawings are, accordingly, to be regarded in an illustrative rather thanrestrictive sense. The claimed inventions are intended to coveralternatives, modifications, and equivalents, which may be includedwithin the spirit and scope of the inventions as defined by the claims.

1. Airborne electromagnetic survey system configured for geophysicalprospecting of an underground formation, which airborne electromagneticsurvey system comprises a transmitter arrangement T_(x) comprising atleast one current source connected to at least one transmitter coilarrangement covering an area A for generating a magnetic moment forbuilding up a primary electromagnetic field inducing a secondaryelectromagnetic field in the underground formation and a switcharrangement comprising at least one switch inserted in between the atleast one current source and the at least one transmitter coilarrangement and configured to connect the at last one current sourcewith the at least one transmitter coil arrangement to build up aconnecting secondary electromagnetic field, which connect is at aconnection time at a connection voltage at a connection current; and todisconnect the at least one current source from the at least onetransmitter coil arrangement to build up a disconnecting secondaryelectromagnetic field; which disconnect is at a disconnection time at adisconnection voltage at a disconnection current; which connectingsecondary electromagnetic field and disconnecting secondaryelectromagnetic field provide a combined secondary electromagneticfield; a receiver arrangement R_(x) comprising at least one receivercoil configured to receive the combined connecting and disconnectingsecondary electromagnetic fields; wherein the airborne electromagneticsurvey system comprises multiple transmitter arrangements T_(x) andcomprises a switch arrangement controller configured to control eachswitch arrangement to disconnect each current source from eachtransmitter coil arrangement at substantially the same time generatingindividual secondary electromagnetic fields whose sum generates a totalsecondary electromagnetic field.
 2. The airborne electromagnetic surveysystem according to claim 1, comprising at least two transmitter coilarrangements that essentially are coupled by being placed on top of eachother.
 3. The airborne electromagnetic survey system according to claim1, wherein the switch arrangement controller is configured to record thedisconnection time of the disconnect by the switch arrangement.
 4. Theairborne electromagnetic survey system according to claim 1, wherein theat least one current source comprises multiple current sources, and theat least one transmitter coil arrangement comprises multiple transmittercoil arrangements, and wherein the switch arrangement controller and isconfigured to control the switch arrangement to respectively connect thecurrent sources to the transmitter coil arrangements at substantiallythe same time.
 5. The airborne electromagnetic survey system accordingto claim 1, wherein the airborne electromagnetic survey system isconfigured to provide multiple combined secondary electromagnetic fieldsthat are essentially identical.
 6. The airborne electromagnetic surveysystem according to 1, wherein the switch arrangement controller isconfigured to record the connection time of the connect by the switcharrangement.
 7. The airborne electromagnetic survey system according toclaim 1, wherein the at least one current source comprises only a singlecurrent source.
 8. The airborne electromagnetic survey system accordingto claim 1, wherein the at least one current source comprises acapacitor, a super capacitor, a mechanical storage, a flywheel, or abattery; or a bank of such.
 9. The airborne electromagnetic surveysystem according to claim 1, wherein the at least one current sourcecomprises a motor generator or a fuel cell.
 10. The airborneelectromagnetic survey system according to claim 1, wherein the switcharrangement comprises at least two switches.
 11. The airborneelectromagnetic survey system according to claim 1, wherein the switcharrangement is arranged with switches in a bridge configuration.
 12. Theairborne electromagnetic survey system according to claim 1, wherein theat least one switch is a semiconductor type switch.
 13. A method ofgeophysical prospecting comprising use of an airborne electromagneticsurvey system configured for geophysical prospecting of an undergroundformation, the method of geophysical prospecting comprising generating atotal secondary electromagnetic field comprising a sum of individualcombined secondary electromagnetic fields using multiple transmitterarrangements T_(x) which transmitter arrangement T_(x) comprises atleast one current source connected to at least one transmitter coilarrangement covering an area A for generating a magnetic moment forbuilding up a primary electromagnetic field inducing a secondaryelectromagnetic field in the underground formation by switching a switcharrangement comprising at least one switch inserted in between the atleast one current source and the at least one transmitter coilarrangement and configured to connect the at last one current sourcewith the at least one transmitter coil arrangement to build up aconnecting secondary electromagnetic field, which connect is at aconnection time at a connection voltage at a connection current; and todisconnect the at least one current source from the at least onetransmitter coil arrangement to build up a disconnecting secondaryelectromagnetic field: which disconnect is at a disconnection time at adisconnection voltage at a disconnection current; receiving a combinedconnecting and disconnecting secondary electromagnetic field using areceiver arrangement R_(x) comprising at least one receiver coilconfigured to receive the combined connecting and disconnectingsecondary electromagnetic fields; and wherein switching of each switcharrangement comprises: disconnecting the at least one current sourcefrom the at least one transmitter coil arrangement to build up adisconnecting secondary electromagnetic field; which disconnecting is ata disconnection time at a disconnection voltage at a disconnectioncurrent; which disconnecting of each current source from eachtransmitter coil arrangement is at substantially the same time.
 14. Themethod of geophysical prospecting according to claim 13, whereinswitching of each switch arrangement comprises: connecting the at leastone current source to the at least one transmitter coil arrangement tobuild up a connecting secondary electromagnetic field; which connectingis at a connection time at a connection voltage at a connection current;which connecting of each current source to each transmitter coilarrangement is at substantially the same time.
 15. The method ofgeophysical prospecting according to claim 13, wherein at least twotransmitter coil arrangement essentially are coupled by being placed ontop of each other.
 16. The airborne electromagnetic survey systemaccording to claim 1, wherein the switch arrangement controller isconfigured to record each disconnection time of each disconnect by eachswitch arrangement.
 17. The airborne electromagnetic survey systemaccording to claim 1, wherein the switch arrangement controller isconfigured to control each switch arrangement to connect each currentsource to each transmitter coil arrangement at substantially the sametime.
 18. The airborne electromagnetic survey system according to claim1, wherein the switch arrangement controller is configured to recordeach connection time of each connect by each switch arrangement.